The present invention relates to wind-energy harvesting machines (for simplicity called wind generators or wind turbines throughout this document) installed off-shore, in particular at sea, to support structures forming parts of such wind generators, and to methods of manufacturing, transportation, installation, maintenance, and protection in stormy weather of such wind generators.
The technical field of the invention is that of wind generators' structure, manufacturing, transportation, installation, maintenance, and protection of such wind generators for harvesting wind-energy off-shore and converting it into electricity or other useful forms of energy.
The majority of the population in the USA is concentrated along coastal lines representing 78% of the total USA electrical power consumption. When looking at harvesting wind energy resources, current proposed solutions are focused on land-based power generation mainly in the Great Plains, and off-shore shallow-water power generation. Both scenarios polarize proponents and critics alike.
A recently published US DOE report titled “20% Wind Energy by 2030” (DOE Report) provided the majority of background information for this paper. DOE report puts an emphasis on land-based power generation.
The Great Plains offer ample wind, yet they are hundreds and thousands of miles removed from the main demand centers—a condition that makes it impossible to utilize all this free wind without massive upgrades to national electrical grid system at a cost of Billions.
Historically wind turbines have been built mostly for land-based power generation. Commercial size wind turbines have a nameplate capacity of 1 to 5 MW (million watts). They are tall structures as high as 70-120 meters with rotors of 70-100 meters in diameter, and of considerable weight: 250-500 metric tones each. There are three major parts: hub, rotor and tower. The hub is placed on top of the tower and holds the machinery needed to produce electricity: an electric generator, gear transferring rotor movement to the generator, a yaw mechanism rotating the hub on top of the tower toward wind direction, and other parts regulating, monitoring and controlling the operation. Erection of a utility-scale wind turbine is a major undertaking due to the dimensions and weight of the main components which are so large as to require specialized oversized vehicles for transportation to a deployment site and even more extraordinary and huge hoisting equipment. Equally difficult and costly is turbine maintenance that sometimes involves replacement of major components.
Nameplate capacity is only achievable at high wind speed of 12-14 meters/second; at wind speeds below this level, the turbine produces only a fraction of nameplate power. For example: at a speed of 7.0 m/s it generates about 35% of nameplate power (for a 1.6 MW turbine that means that only about 550 kW (thousand watts) are generated, not 1.6 MW), at a speed of 8.5 m/s the fraction is about 60% (the same turbine generating 950 kW). This fraction is called a capacity factor CF. CF is proportional to wind velocity at power of 3; therefore a seemingly small increase in wind speed causes big differences in the power produced. At wind speed of 20-25 m/s a brake is activated stopping the rotor as a measure preventing damage to the turbine—power generation ceases.
Federal agencies have prepared maps depicting the distribution of wind resources for the continental United States and sea areas along the coasts. The maps use wind classification as follows: wind class 1 & 2—average annual speed at 50 in above ground 0-5.6 m/s (considered marginal, not suitable for commercial generation), class 3: 6.4-7.0 m/s at 50 m height, class 4: 7.0-7.5 m/s, class 5: 7.5-8.0 m/s, class 6: 8.0-8.8 m/s, class 7: >8.8 m/s. The USA major land areas suitable for commercial power generation are of class 3 and 4 with 7.0 m/s average annual wind speed. The DOE Report shows that in order to achieve 20% of electrical power from wind by 2030, about 300 GW (billion watts) of nameplate capacity should be installed. The DOE Report assumes that the majority of wind turbines will be built on land where the average wind speeds are 7.0 m/s and the CF=35%. The Report expects CF to grow to 41% by 2030 due to technological improvements; therefore the actual power delivered from wind farms of nameplate 300 GW will be about 125 GW.
Offshore wind is classified as class 6 (the average annual speed at height of 50 m is 8.5 m/s for class 6) and in some areas class 7. Currently at this speed CF equates to 60%, and by 2030 it can be expected to increase to 70% in the same proportion as at lower speeds. Even if CF equates eventually only 65%, the production of 125 GW at sea will require installation of less than 200 GW nameplate capacity—33% reduction when all energy is produced at sea instead of land. The DOE Report gives a figure of 400 GW available offshore wind resource that can be fed into the existing transmission grid. This amount should be sufficient by itself to satisfy the 20% renewable energy goal and more.
Shallow water off-shore power generation does address the need of generating power near demand centers, yet it requires foundations to be built at sea upon which turbines are installed. The difficult nature of turbine erection at sea increases the cost of deployment by about 42% versus land-based variety. Shallow-water turbines also have galvanized communities to reject such installations on a variety of reasons including environmental, ecological, real-estate and tourism.
The prospects of leveraging deep water off-shore wind power generation have been stymied as the cost of such installations proved to be too high and the engineering challenges to secure a wind-farm in deep waters was too daunting. The DOE Report mentions shallow-water offshore wind energy but gives it minor attention, accentuating as disadvantaged the economics for sea-based generation.
The present invention presents a contrary opinion: a structure and method for wind energy harvesting at sea to become a preferred modality economically and technically in a short period of time.